In conventional optical projection-exposure apparatus for microlithography, the reticle (mask) or substrate typically must be removed from the apparatus in order to ascertain and detect whether or not any contaminant particles are present on the reticle or substrate and, if so, to remove the contaminant particles from the reticle or substrate. After removing the contaminant particles, the reticle is conventionally covered with a protective film such as a film of periclase (MgO) and used while so covered.
With conventional apparatus used for charged-particle-beam (CPB) lithography, such as electron-beam lithography, it is not practical to cover the reticle with a protective film because the charged particles in the beam generally experience difficulty in penetrating the film. Consequently, with such conventional apparatus, the reticle must either be used without cleaning or, if previously cleaned, with any new particles that have become attached to the reticle since the last cleaning.
Since CPB lithography is performed in a vacuum chamber, having to remove the reticle or substrate for inspection and/or cleaning is cumbersome. Also, removing the reticle or substrate from the vacuum chamber increases the likelihood of further contamination of the reticle or substrate.
Unfortunately, images of contaminant particles attached to a reticle can be projected onto the substrate. During such projection, the projected image of the contaminant particles can be magnified with decreases in the desired feature size of the projected pattern. Since a reticle used in a CPB lithography typically cannot be covered with a protective film, contaminant particles present in the CPB lithography apparatus can become attached to an originally clean reticle after the reticle is installed in the electron beam projection apparatus.
Also, as the feature sizes defined by the reticle are made progressively smaller, it becomes increasingly difficult to inspect and remove contaminant particles attached to the reticle.